1. Field of the Invention
The present invention relates to an outboard motor.
2. Description of the Related Art
An outboard motor according to a prior art is disclosed in Japanese Published Unexamined Patent Application No. 2008-309129. The outboard motor includes an engine, a driveshaft, a propeller shaft, a propeller, and a forward-reverse switching mechanism. A rotation of the engine is transmitted to the propeller successively via the driveshaft, the forward-reverse switching mechanism, and the propeller shaft. The engine and the driveshaft are arranged to rotate in a fixed rotational direction. When the rotational direction of the propeller is to be switched from a forward drive direction to a reverse drive direction, the rotation transmitted from the driveshaft to the forward-reverse switching mechanism is reversed, and the reversed rotation is transmitted to the propeller shaft.
Also, the outboard motor includes an engine cover that houses the engine, and a ventilating apparatus that ventilates an interior of the engine cover. The ventilating apparatus includes a fan disposed inside the engine cover, and an air delivery passage. The fan is connected to the air delivery passage. The fan is driven by the engine. The fan is rotated at a rotational speed corresponding to the engine speed. Also, the air delivery passage is defined by a flow passage case disposed inside the engine cover. An exit of the air delivery passage opens at an outer surface of the engine cover. Air taken into the interior of the engine cover by the rotation of the fan is discharged to an exterior of the engine cover through the air delivery passage.
Also, the outboard motor includes an exhaust passage, a catalyst, an air introducing passage, and a check valve. An upstream end of the exhaust passage is connected to the engine. A downstream end of the exhaust passage is connected to the propeller. An exit of the exhaust passage opens underwater. The catalyst is disposed at the upstream end of the exhaust passage. Also, a downstream end of the air introducing passage is connected to the upstream end of the exhaust passage. An upstream end of the air introducing passage is connected to the air delivery passage. The air introducing passage is defined by a dedicated member. Also, the check valve is disposed in the air introducing passage. The check valve is arranged to open and close according to a pressure inside the air introducing passage. The check valve is arranged such that air flows only from the air delivery passage side to the exhaust passage side.
When in a state in which a hull is propelled forward by the outboard motor, the forward-reverse switching mechanism is switched from the forward drive state to the reverse drive state, the propeller may be pushed by a water stream and rotate in the forward drive direction. That is, although the engine rotates the propeller in the reverse drive direction, the propeller is rotated in the forward drive direction by the water stream. The engine is thereby reversed. Air inside the exhaust passage is thus sucked into the engine and the pressure inside the exhaust passage becomes negative (a pressure lower than the atmospheric pressure). The check valve opens when the pressure inside the exhaust passage is negative. Air inside the air delivery passage is thereby supplied into the exhaust passage via the air introducing passage. The negative pressure inside the exhaust passage is thus eliminated. Water entering into the downstream end of the exhaust passage is thus prevented from reaching the upstream end of the exhaust passage and wetting the catalyst.